The present invention relates to the general field of greenhouses of the “span” type that are designed to be installed in multi-span runs each covering several thousand square meters or indeed several hectares uninterrupted.
Each “span” greenhouse is constituted by a plurality of structural units installed parallel to one another over the entire length of the bay.
Each structural unit comprises at least two side vertical posts, separated by a predetermined bay width, and supporting roof-forming arch segments.
In conventional manner, the structural units are connected together at the top ends of the posts via longitudinal gutters.
The invention relates to greenhouses having a roof covering made up of a transparent wall formed by a tensioned film of plastics material of the single wall type or of the inflatable double wall type. Since the use of plastics films as roof coverings for greenhouses has become widespread, the bay widths of the greenhouses manufactured across the world have increased unceasingly.
In parallel with the development of the use of greenhouses with single-walled and then inflatable double-walled plastics roofs, across the world, bay widths have increased, and multi-bay installations have supplanted single-bay installations. Single-bay installations are installations made up of a single span whereas multi-bay installations are made up of more than one span.
Widening the bay width of the greenhouses, and developing the multi-span concept procures a reduction in the raw materials used and thus a reduction in investments. Such installations make it possible to obtain a good return on investment.
The invention is situated in the context of such change. The invention relates more particularly to greenhouses of the “multi-span” type in which the bay width is greater than 9 meters (m) and that are installed in multi-bay installations.
It is recognized by the profession that the optimum profile for the vault of a greenhouse must maximize the extent to which it tracks the angles of incidence of the sun in order to maximize the amount of sunlight it receives. The plastics film must then, as far as possible, be situated at 90° relative to the angle of incidence. An angle not equal to 90° increases the thickness of the plastics layer through which the ray of sunlight passes and reduces the power thereof. This rule has led to almost generalized use of a greenhouse profile having a ridge height over bay width ratio that is always greater than 1 to 5, and, above all, having a pointed arch shape. Such a profile is very effective in preserving optimum angle of incidence with the sun's rays, and thus in limiting the problems of refraction at small angles of incidence.
However, a multi-bay installation implies the presence of shadows cast by each greenhouse vault on the vault of the next greenhouse. This does not take place in single-bay span greenhouses. It can thus be observed that wide zones of shade are cast at the start of the day and at the end of the day, giving rise to delayed arrival of the sunlight and to a premature drop in the sunlight available for photosynthesis in most multi-span greenhouses.
In addition, the more the bay width increases, the longer the time for which a given strip of the greenhouse remains in the shade.
And yet it is increasingly crucial to improve productivity in greenhouses, where such productivity depends mainly on the incoming quantity of sunlight responsible for photosynthesis, at all points of the greenhouse.
The presence of cast shadows has become increasingly counterproductive as spans have widened, because of the sizes of the shadows cast and because of the sizes of the resulting masses of cold air.
Since the results obtained remain satisfactory, the generally high pointed-arch shape is little contested by the profession. The recent arrival on the market of greenhouses having bay widths greater than 12 m has, however, rekindled questions about the presence of the cast shadows.
Another very important point in the operation of greenhouses of bay width greater than 9 m is the effectiveness of their ventilation. It is currently recognized that the presence of double opening windows along the ridge of the greenhouse is an optimum solution, particular in regions with little wind.
The presence of such central double opening windows along the ridges of greenhouses of large bay width is particularly advantageous because such windows make it possible to stimulate the effects of air convection, in particular at the start of the day when the problems of non-uniformity of temperatures due to the presence of cast shadows are particularly significant. Locating such double opening windows at the tops of the spans is crucial in order to generate movements of air throughout the space inside the greenhouse, above all in the absence of wind.
The air convection movements are thus triggered by a chimney effect. The taller the greenhouse, the more effective the chimney effect is. The invention thus concerns greenhouses for which the vertical posts are of height greater than 3.5 m.
In windy regions, other locations for opening windows make it possible to blow enough air through to cause the air at the tops of the bays to flow. Such a greenhouse is known that has an opening window along the gutter and that has a ridge height to bay width that is very small and less than 0.15.
This profile has been criticized because it was then observed that, whenever the ratio of the distance between the top of the greenhouse and the straight line passing through the top ends of the side posts over the bay width is less than 1 to 5, the presence of condensation becomes prejudicial substantially over the central third of the vault.
With such a dimension ratio, condensation cannot be removed by gravity along the walls of the vault. In the absence of wind, the air present at the top of the greenhouse cannot be removed, condensation is thus persistent and generates a problem of dripping. Such phenomena are observed particularly at the start of the day when the start of the photosynthesis cycle generates evapotranspiration of the plant matter that is five times higher than during the night, and a sudden increase in humidity. These phenomena are a recognized problem of vaults of low characteristic dimensional ratio.
For a characteristic dimensional ratio less than 0.20, the angle from which the ridge is seen from the top end of the post is less than 21.8°. It is also necessary for the arch segments forming the roofs of greenhouses covered with plastics walls to have a minimum curvature taking their path away from the straight line connecting the ridge of the greenhouse to the top end of the post. This minimum curvature of the roof-forming arch segments, even with the use of an arcuate segment or “arc” that is of broken curvature or that is pointed arch shaped, makes it possible to access a maximum angle at the top of the greenhouse that is less than 15° for bay widths exceeding 9 m. That angle does not allow drops to run off along the walls. Condensation phenomena are thus very problematic in greenhouses having large bay widths, side opening windows, and a characteristic dimensional ratio less than 1 to 5.
It is then necessary to wait either for action from wind to dry the atmosphere or for the inside of the greenhouse to heat sufficiently to see the condensation problem disappear.
Installing an opening window on the ridge in order to allow central aeration is precluded because the phenomenon of condensation on the low-gradient central portion would still not be prevented, in particular at the start of the photosynthesis cycle, and the opening movements would give rise to even more significant dripping.
Opening the central opening window at the start of the day would also be disastrous because the movements to open the opening window on which the condensation at the top of the greenhouse has built up would systematically give rise to dripping. Such dripping can take place starting from a low point of the pouch formed by the bottom wall of the inflatable double wall or can take place starting from any point of said bottom wall.
The invention proposes to solve this particular problem of condensation for greenhouses having flattened-arch profiles. The invention also proposes to sweep aside a preconception about the ineffectiveness in terms of performance of greenhouses having flattened-arch profiles.